In my case there were active directory and mailbox disabled users, that were listed as allowed senders for distribution group. When tried to add new users as allowed senders for distribution group, there was error message:

Error: Couldn't find object "domain/ou/disabled user". Please make sure that it was spelled correctly or specify a different object.
Resolution for this problem is to remove disabled users as allowed senders, which can't be seen from Exchange Management Console 2010. One way to do that is to enable advanced feature in active directory users and computer console and edit the authOrig attribute from Attribute Editor. Remove all disabled accounts, refresh Exchange Management Console and you will be able to add new users as allowed senders for distribution group.